The present invention relates to a power transmission device for four wheel drive vehicles, and particularly to an improvement in the structure of the center differential gear in a power transmission unit for full-time four wheel drive vehicles, which transmits rotation to both front and rear wheels at all times.
Heretofore, there have been proposed various kinds of power transmission devices for four wheel drive vehicles, which are obtained by slightly modifying the power transmission devices for transverse front engine front wheel drive vehicles.
In the above mentioned conventional devices, as shown in FIG. 3, drive power from the engine transmitted through a hollow shaft 45' to a differential gear carrier (diffcarrier) 41' which is rotatably supported on the shaft sections 42'a, 43'a of both side gears 42', 43' of the center differential gear 33', is distributed among both side gears 42', 43' from pinion 50' on the diff-carrier 41'. The drive power transmitted to the left hand side gear 42' is transmitted through another hollow shaft 46' to the front wheel differential gear (not shown) and distributed among side gears to be transmitted to the front axles on both sides (only the right front axle 31' is shown in FIG. 3.). The drive power transmitted to the right hand side gear 43' is transmitted through a ring gear mount casing 32' and a rear-wheel drive ring gear 35' to a gear 40' and then transmitted from a drive pinion-shaft 39' through a propeller shaft and a final speed reduction gear (not shown), to be distributed among the rear axles on both sides. Further, in the above-mentioned transmission of drive power, axial forces generated at side gears 42', 43' are transmitted respectively to the diff-carrier 41' and the ring gear mount casing 32' through thrust washers 51', 52', 53', 55' to be borne by the carrier 41' and the casing 32'.
The above-mentioned conventional power transmission device has a disadvantage in that, if the dimensions of the side gears 42', 43' are increased in order to withstand larger input torques, not only does the outer diameter of the diff-carrier 41' but also the diameters of the ring gear mount casing 32' and transfer casing 12' become larger, so much so that the transfer casing 12' can no longer be contained within the restricted space available behind the engine.
Accordingly, an object of the present invention is to provide an improved structure for a center differential gear in a power transmission device for a four-wheel drive vehicle, in which the diff-carrier is supported on the shaft section of the side gear on the front axle drive side and not on the shaft section of the other side gear transmitting power to the ring gear mount casing, thereby eliminating the above-mentioned problem and allowing the side gears to be of optimum size even within a confined space.